1. Field of the Invention
The present invention relates to an apparatus for the irradiation of a primary particle beam and a method for irradiation of the same, more particularly, to an apparatus for the irradiation of a primary particle beam and a method for irradiation of the same wherein a primary particle beam such as an ion beam is irradiated on a target such as a semiconductor wafer placed on a rotating disk, with a certain width of scanning, for ion implantation etc.
2. Description of the Related Art
In general, when performing ion implantation on targets such as semiconductor wafers, for example, when controlling the threshold value Vth of a MOS transistor, the dosage of the ions to be implanted into the semiconductor wafers is low and the current of the ion beam, in accordance with that low dosage, is kept at a low current of from 0.1 microamperes to 1 or 2 microamperes, thereby reducing the amount of heat given to the wafers.
As opposed to this, when a source or drain region of a MOS transistor is formed, for example, the dosage of the ions to be implanted into the semiconductor wafers is high. Therefore, to give the same processability as with the above control of the Vth, from the viewpoint of the production of the integrated circuit, the current of the ion beam, in accordance with this high dosage, must be made a large one of about 10 milliamperes. As a result, the power, or heat, given to the wafers (expressed as the ion beam current.times.the ion beam acceleration voltage) reaches, for example, about 10 milliamperes.times.200 kV=2 kW.
Therefore, in the former implantation of ions (for the Vth control), the wafers are fixed in place and an ion beam of a certain strength is raster scanned on the wafers, whereby even if an ion beam of a certain strength is irradiated for each wafer by a single raster scanning, the amount of heat given to the wafers is small, so there is relatively less of a rise in the temperature and other problems are avoided.
However, in the latter implantation of ions (for the formation of source and drain regions) in the wafers, a plurality of wafers are arranged on the side of a rotating drum or on the surface of a rotating disk, an ion beam of a certain strength is scanned successively on the plurality of wafers so as to cover at least the wafer regions, and the scannings are repeated a plurality of times (for example, from a few times to about a 100 times, in accordance with the dosage), whereby the need arises to disperse the heat instantaneously given to the wafers.
In this case, compared with the case of use of the above rotating drum, use of the above rotating disk is better in that, due to the rotary mechanism, it is possible to improve the rotating speed of the disk and is easier by that much to disperse the power (heat) given to the wafers. Therefore, the larger the power to be given to the wafers, the more advantageous the use of a rotating disk.
That is, in this case, a plurality of wafers are arranged on the rotating disk along its circumferential direction and an ion beam of a certain strength is irradiated over the entire surfaces of the plurality of wafers by irradiating the ion beam reciprocatively across a certain distance (a distance slightly over the width of the diametric direction of the wafer) (therefore, due to the rotation of the disk, the ion beam follows a path like along the groove of a record to successively irradiate the plurality of wafers, whereby predetermined regions including the plurality of wafers are successively irradiated). The reciprocative irradiation of the ion beam (scanning) is repeated a plurality of times, as mentioned above, whereby the dosage of the ions implanted in the wafers becomes the predetermined high dosage. Note that in this case, instead of reciprocatively moving the ion beam a plurality of times by a predetermined width, as mentioned above, it is conceivable that the rotating disk be reciprocatively moved a plurality of times by the predetermined width. In general, however, the former technique enables high speed reciprocative movement and, therefore, usually the former means is adopted. Note that in reciprocatively moving the ion beam by the predetermined width, the ion beam is moved successively in steps with each predetermined width, that is, if the width of movement per step in the scanning is .DELTA.W, by steps with each (.DELTA.W).
However, in irradiating such an ion beam, differences arise in the area which is implanted, on the rotating disk, at the inner circumferential side and outer circumferential side of the irradiation regions including the plurality of wafers (in the radial direction of the disk). Therefore, wen the implantation time (residence time) of the ion beam at each step is made equal, the dosage of the ions at the inner circumferential side (per unit area) becomes larger than the dosage of ions at the outer circumferential side (per unit area), resulting in nonuniform dosages per unit area and making necessary changes in the speed of movement of the ion beam (in other words, residence time of the ion beam with each step). In this case, there would be no problem if the waveform of the scan signal for changing the irradiation position of the ion beam were changed synchronously to a predetermined waveform, in accordance with the change in the residence time, so as to make the dosage uniform over a predetermined width of irradiation region, but in actuality the magnitude (so-called "gain") and offset of the scan signal cause variations in the irradiation width of the ion beam and positional deviations in the irradiation range of the ion beam (over the predetermined width). Further, the nonlinearity of scan magnets etc. used for controlling the path of irradiation of the ion beam (for example, due to the shape of the magnet or the edging effect generated in the magnet) cause the magnetic field intensity of the scan magnets (that is, the position of irradiation of the ion beam) to fail to change linearly with respect to the scan signal, which in turn causes deviation of the irradiation width and irradiation range of the ion beam. Therefore, the uniformity of the dosage of the ion beam on the wafers deteriorates and, further, the reproducibility of the dosage on the wafers for different batches deteriorates.
In view of these problems, in the prior art, the difference in the implantation area in the radial direction of the disk and the nonlinearity of the scan magnets have been corrected by, for example, using test wafers and making a map of the ion implantation amount in the radial direction (for example, measuring this by the sheet resistance) and by using a predetermined scan signal programmed in advance based on the map to correct, by software, the magnetic field intensity or the amount of change of the magnetic field of the scan magnets.
However, even with this method, in actuality the irradiation position of the ion beam irradiated on the wafers (which strictly speaking differs with each operation of the apparatus, that is, with each batch) is not detected and, therefore, strictly speaking, it is not possible to sufficiently correct the nonuniformity of dosage arising from the differences in implantation area and nonlinearity of the scan magnets, which differ with each operation of the apparatus.